The present invention relates to safety devices used in conjunction with high speed centrifuge devices and, more particularly, to a method and apparatus for detecting the absence of a coded disk fixed to the base of the rotor to indicate the maximum design speed of the rotor. In present centrifuge systems, the rotor may be changed to permit the use of one type of rotor for high speed while another type, usually less expensive, for low speed applications. Overspeed indicators are provided as a safety device on each rotor to indicate its maximum design speed. Centrifuge rotors, as well as any rotating devices, can be extremely hazardous when driven at a rotational speed in excess of their design maximum. It is well known in the art that a centrifuge rotor driven in excess may explode due to high centripetal and centrifugal forces. To prevent this occurrence, coded disks are fastened to each rotor to indicate its maximum safe rotational speed.
Prior art has disclosed several systems for determining the presence of an overspeed disk, all of which have deficiencies.
One system is to use a single coded disk on the rotor to indicate rotational speed when used in conjunction with a tachometer. In this system the disk consists of highly polished and darkened areas. At startup, the sensor will determine the intensity of the light reflected from the disk. This system has its deficiency in that should the rotor stop at a point where a darkened area is opposite the light source, zero light intensity will be received by the detector. Furthermore, should the disk have fallen off, the natural reflective qualities of the metal below the disc could reflect enought light to indicate the presence of the overspeed disk on the centrifuge startup.
Another system, which also uses a single coded disk, operates on the time between darkened areas on the disk. Should the time difference between darkened areas be too great during startup, a protective circuit will disable the centrifuge. This system has its deficiency as an outgrowth of the manner in which overspeed disks are fixed to the rotor. Overspeed discs adhere to the rotor by the use of an adhesive. Should the disk fall off, portions of the rotor will reflect light through the natural reflective properties of the metal rotor while other portions will disperse the light due to adhesive residue. As such, the adhesive could block the reflecting of the light in such a manner as to indicate the presence of an overspeed disk. When this occurs, the centrifuge will operate without the safety of an overspeed indication and may exceed the maximum design speed of the rotor.
A third type of system which prior art has disclosed consists of two concentric coded disks, one of which is used to indicate overspeed while the other indicates rotational speed. The disk used as an indication of overspeed comprises a donut-shaped disk having a darkened area and a light reflecting area. The light reflecting area is proportionate to the maximum design speed of the rotor in use. By lengthening the area which is light reflective, the higher maximum design speed of the rotor is indicated. A light source transmits light to the reflective area which issues a pulse having a length proportionate to the reflective area. The leading edge of this reflective area pulse triggers a constant pulse generated by associated circuitry. The reflective pulse is compared with the constant pulse to determine whether the rotor maximum design speed has been exceeded. In the absence of the overspeed disk, the natural reflective properties of the metal rotor will indicate capability for an infinite speed and never trigger the constant pulse for comparison. Should some adhesive remain behind triggering the constant pulse for comparison, a reflective pulse indicating a maximum design speed greater than the true maximum design speed of the rotor may be present.
These prior art systems can be found in U.S. Pat. No. 3,436,637 to Robert J. Ehret; in U.S. Pat. No. 3,921,047 to Alan W. Carter et al. and in U.S. Pat. No. 3,832,614 to David W. Olliffe. Additional prior art methods for overspeed detection and rotational speed determination can be found in U.S. Pat. No. 3,691,440 to Logan D. Haddock; U.S. Pat. No. 3,809,982 to Kenneth J. Cook; U.S. Pat. No. 3,777,233 to Luc Yves Natens; U.S. Pat. No. 3,970,245 to Hans-Peter Aeschlimann; U.S. Pat. No. 3,746,247 to John Camilliere; U.S. Pat. No. 3,731,301 to John Christopher Davis; U.S. Pat. No. 3,462,670 to W. E. Waye; U.S. Pat. No. 3,582,699 to Rosario S. Badessa and in U.S. Pat. No. 3,292,068 to E. J. Holden. Although the foregoing patents disclose systems for overspeed and rotational speed determination, they all have common deficiencies in that the loss of an overspeed indicator disk means the loss of the safety of an overspeed protections system.